In contrast, law figured prominently in a separate briefing on the conduct of the strikes, the specific targets, the precautions taken to avoid civilian casualties—indeed, Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford’s detailed recitation offered a veritable catalogue of law of war obligations with which the United States has complied.

Both the resort to force—the “why”—and the conduct of hostilities—the “how”—must be lawful under international law. Nonetheless, here it appears that why the United States used force is a question of legitimacy, while how the United States uses force is a question of lawfulness. The difference between the two is important—not only for what it reveals about the authority to launch the strikes, but as a new step in the long-standing and often inseparable dance between legitimacy and lawfulness in the context of military operations.

To anyone who saw the photos of the aftermath of the attacks, with at least 70 killed and countless children gasping for air, destroying Syria’s chemical weapons capability and punishing the regime for its continued use of chemical weapons against its own population in flagrant violation of international law surely seems like a textbook example of “justified” or “legitimate.” After all, as both Defense Secretary James Mattis and Pentagon spokesperson Dana White reiterated countlesstimes, no civilized nation can or should tolerate the use of chemical weapons.

This legitimacy, however, does not derive from international law. Although international law flatly prohibits the use of chemical weapons—in wartime or peacetime—international law also prohibits the use of force by one state against another. This ban is the central foundation of our international system. The only exceptions are self-defense, a U.N. Security Council authorization, or the consent of the territorial state concerned. (Although a few states, including the United Kingdom, recognize a narrow exception for humanitarian intervention, the United States does not, and there is no international consensus that such an exception is accepted international law.) Notwithstanding the moral imperative that chemical attacks might generate, retaliation or punishment for the use of chemical weapons, or deterrence against the future use of such weapons, are not lawful reasons to use force.

In fact, there is no international legal authority for the strikes, as the deafening silence from the Trump administration regarding an international law justification for its actions attests. The administration has rightly condemned Syria’s violations of international law, but it has instead turned to the language of justness and moral outrage to justify its use of force, pitting the allied forces’ “righteous power” and “noble warriors” against Syria’s “barbarism and brutality.”

The United States is thus using legitimacy in a four-step effort to create lawfulness. Step one: Catalogue and denounce Syria’s extensive violations of international law, which by now are too numerous to count. Step two: Affirm the need for accountability for violations of international law, surely critical for any effective enforcement of international law and deterrence for future violations. Step three: Harness the universal moral outrage at the horror of last week’s chemical weapons attacks and seven years of unending brutality against civilians and the desire to “make Assad pay” for what he has done. Step four: Add moral legitimacy to international law violations and the need for accountability, and the result is an appearance of lawfulness.

This rhetorical tactic has proved quite effective, as evidenced by the glaring absence of questions or reporting on whether the U.S. and its allies complied with international law in the resort to force against Syria. However, beyond this messaging success, there is now a new step in the pas de deux between legitimacy and lawfulness that is as old as war itself.

Legitimacy has always been an essential component of military operations, particularly with regard to public support for both the launch and continuation of such operations. Although the lawful resort to force was once the primary key to legitimacy, in recent years, compliance with the law of armed conflict—namely the principles of distinction, proportionality and precautions—in the conduct of military operations has become the central pillar of legitimacy.

Even a cursory glance at the discourse on military operations demonstrates this link between compliance with the law of armed conflict and legitimacy. In today’s world of nearly instantaneous media and social-media coverage of military operations even in the farthest reaches of the globe, civilian casualties and the mere perception of war crimes can drastically undermine legitimacy both at home and abroad. In many military operations, such as counterinsurgencies, protection of the civilian population is central to mission success, and therefore compliance with legal rules designed to protect civilians is essential for legitimacy. Similarly, a military that is or appears to be committing war crimes may lose legitimacy at home, eroding valuable public support necessary to sustain military operations. Finally, compliance with the law of armed conflict is the centerpiece of legitimacy in the international community, such that violations can undermine cohesion and diplomatic efforts among the coalition. For these reasons and many more, the United States and its allies go to great lengths to demonstrate their adherence to the fundamental principles and rules of the law of armed conflict.

Lawfulness has thus been the touchstone for legitimacy, whether in the form of compliance with the international law governing the resort to force, historically, or compliance with the law of armed conflict, in today’s operations. But legitimacy is now being used as the measure of lawfulness in the absence of actual compliance with the law. Obfuscating the lack of international legal authority for Friday’s strikes is, of course, the immediate consequence. After all, the combination of Syria’s atrocities and U.S. moral justifications seems to have done the trick—moral legitimacy may not merely be substituting for lawfulness here (such as the “illegal but legitimate” description of the 1999 NATO bombing of the former Yugoslavia), but actually appears to be creating lawfulness.

But a far more damaging consequence may well be a steady erosion of law and legality in favor of legitimacy alone. Legitimacy is essential, but it must rest on law—not righteousness, political imperatives, religion, shared cultural ties, or the exigencies of a given moment. No less than the stability and predictability of our international system—and the ultimate legitimacy of U.S. actions—is at stake.

Laurie R. Blank is a clinical professor of law and director of the International Humanitarian Law Clinic at Emory University School of Law, where she teaches international humanitarian law and works directly with students to provide assistance to international tribunals, non-governmental organizations, and law firms around the world on cutting-edge issues in humanitarian law and human rights.